1. Field of the Invention
This invention relates to fluorinated hydrocarbon polymers and in particular to polytetrafluoroethylene (PTFE).
2. Description of the Prior Art
Electrodes for use in electrochemical cells such as fuel cells are well known in the art. One common type of electrode is the gas diffusion electrode. A pair of gas diffusion electrodes are disposed on either side of a compartment containing a matrix which is soaked with electrolyte. Catalyst is disposed on the electrolyte facing surface of each electrode. Hydrogen is fed to the back side of one electrode while oxygen or air is fed to the back side of the other electrode. The gases enter the electrodes and react with the electrolyte in the presence of the catalyst.
One type of gas diffusion electrode comprises a layer of PTFE combined with a catalyst supported on carbon particles (i.e., catalyzed carbon), the layer being disposed on the surface of a porous carbon substrate (e.g., carbon paper). The PTFE (or any other compatible hydrophobic polymer) prevents the electrolyte from filling up the electrode to such an extent that sufficient fuel or oxidant cannot reach the catalyst. A gas diffusion electrode of this type is described in column 5 of commonly owned U.S. Pat. No. 3,857,737 to Kemp et al.
In order to obtain good performance from the fuel cell it is critical that the PTFE and the catalyzed carbon be intimately mixed, highly dispersed, and uniformly distributed throughout the PTFE/catalyst layer. One technique for applying such a layer to a substrate is the direct filtration method. In that method catalyzed carbon powder and an aqueous PTFE dispersion containing colloidal size PTFE particles are blended into an alcohol/water solution to form a co-suspension of carbon and PTFE therein. The suspension is caused to floc, such as by heating or adding a floccing agent. Floccing is the coalescence of the catalyzed or uncatalyzed carbon particles with the PTFE particles. After floccing, excess liquid is decanted and the floc is applied to the surface of a carbon paper substrate which acts as a filter. Liquids pass through the substrate and a layer of the desired solids mixture (i.e., the floc) remains on the surface of the substrate. The article is dried, compacted, and sintered. The resulting layer has all the requisite properties discussed above, and electrodes may be made by this method which perform very well.
The foregoing technique and other known techniques for forming good electrodes are slow, expensive, and not suitable for production runs wherein hundreds and thousands of electrodes must be made in a relatively short period of time at minimal cost.